「一家三口,父亲半价」的英文翻译可参考以下：
Family of three,and half price for the father.
Family of three; half price for the father.
Family of three,and 50% off for the father.
Family of three; 50% off for the father.

Family of three,and 50 percent discount for the father.
Family of three; 50 percent discount for the father.


50% 或 50 percent 皆可,正式一点就是用 50 percent,它们的意思与直接讲 half price 「半价」一样.